Biomimetic dry adhesives are inspired by the fibrillar structures found on the feet of geckos and certain spiders. These adhesives have been investigated by multiple research groups for use in applications ranging from climbing robots, to use in surgical tools or bandages, for example. Microstructuring surfaces into fibers has been shown to allow relatively stiff materials to be more compliant in order to make intimate contact with substrates so that van der Waals interactions can produce significant adhesion for exploitation in dry adhesive structures. One application is to develop biomimetic dry adhesives for use in space applications. Potential advantages of these types of adhesives for use in space is that dry adhesives may provide for operation in vacuum without problems of out-gassing encountered with traditional pressure sensitive adhesives (PSAs), and could potentially be used on almost any surface. While the structures of gecko feet have been confirmed to operate in vacuum, there have been conflicting reports on the effectiveness of synthetic dry adhesives in vacuum conditions. This is partially due to the higher number of synthetic dry adhesive designs that use mushroom shaped fibers—an innovation that could potentially introduce a suction cup effect which may fail in low-pressure environments.
Biomimetic dry adhesives with mushroom shaped fibers have been found to be far more effective than their flat tipped counterparts for loading in the normal direction. While multiple groups have tested high aspect ratio fibers made of stiff polymers or carbon nanotubes, these adhesives generally perform much better in shear than with normal loads. In contrast, softer materials with mushroom shaped fibers demonstrate normal adhesion that is much greater than unstructured surfaces, and can have a high ratio of adhesion strength to pre-load. Multiple research groups have developed methods of producing mushroom shaped adhesive geometry, with fiber diameters ranging from <5 to >50 μm. In theory, these fibers operate primarily on van der Waals interactions between surfaces, and may operate effectively under vacuum. In practice, several groups have reported on performance degradation under low pressure conditions, or adhesion underwater—an unexpected occurrence if van der Waals forces are the primary cause of adhesion. In some such reports, the caps on the pillars were large (>40 μm) but no systemic investigation on the effectiveness of fibers with different cap sizes and at different pressures has been demonstrated before now. In an embodiment of the present invention, the microscale adhesion properties of elastomer based synthetic dry adhesives with a variety of cap diameters are examined.
In other applications which do not require adhesion of the dry adhesive in low-pressure environments, effective adhesives made of relatively soft materials (E˜1-10 MPa) have shown in experimental results that the shape of the fiber tip itself is dominant when determining maximum adhesion pressure, with mushroom shaped tips demonstrating the greatest effectiveness. Offset caps have been demonstrated such as by dipping and smearing flat fiber tips in fresh silicone, but their measured adhesion was less than that of aligned mushroom caps. More recently, angled tips have been used by different research groups to replicate some anisotropic behavior but these methods have required complex lithography or dipping techniques to define the molds or produce the final directional dry adhesives.